Most of the permanent magnetic lifting devices on the current market have a 3 time safe lifting coefficient under ideal conditions. For example, if a permanent magnetic lifting device has a nominal lifting capacity of 250 kg, its maximum clamping force can be 750 kg under ideal conditions, that is, the maximum clamping force generated under ideal conditions can be 3 times nominal lifting capacity. This is what we call safe lifting coefficient. Because the maximum clamping force that can be generated by a permanent magnetic lifting device relates to many factors, such as the material, dimension, and surface condition of a workpiece, and the size of contact area between a workpiece and a permanent magnetic lifting device, the maximum clamping force that can be generated by a permanent magnetic lifting device may be different under each specific condition. It is possible that a permanent magnetic lifting device with the nominal lifting capacity of 250 kg may generate a maximum lifting force lower than 250 kg under a certain condition. Because an operator has no way to know the maximum clamping force that can be generated by a permanent magnetic lifting device with a certain nominal lifting capacity, even though a workpiece weighs less than nominal lifting capacity, and the workpiece can be lifted up, its safe running is not assured. This is because that the lifting device may be in the critical condition that the workpiece is just workpiece may fall, resulting in a potential risk in safety.
Therefore, if an operator knows whether the ratio of the maximum clamping force that can be generated by a permanent magnetic lifting device under each specific condition to the weight of a workpiece equals or exceeds a certain value, the operator will know whether it is safe to lift the workpiece under this specific condition. What the ratio should be to ensure safety depends on the operation parameters of the lifting mechanism of the permanent magnetic lifting device. This is not covered in the scope for discussion in this patent. However, generally speaking, this ratio can be set to 2, that is, when the ratio of the maximum clamping force that can be generated by a permanent magnetic lifting device to the weight of a workpiece equals or exceeds 2, potential risk in safety can be basically eliminated.